1. Field of the Invention
This invention is concerned with down-hole acoustic logging tools for use in vertical seismic profiling.
2. Discussion of the Prior Art
In vertical seismic profiling, a seismic sensor is lowered into a bore hole to a plurality of predetermined depths. At each depth station, a surface source generates a seismic wavefield. The seismic waves detected by the sensor at the respective depths are transmitted by suitable means to data storage and processing means of any well known type.
Typically, the sensor is mounted on or associated with some sort of sonde which may include other tools such as an inclinometer to measure the slant of the bore hole. The sonde may be one or two meters long and six to ten centimeters in diameter and may weigh on the order of 100 kilograms. The sonde is maneuvered up and down the borehole by a cable connected to a winch located at the surface. At each depth station, the sonde is locked firmly in the hole by a mechanical caliper device of known type that is usually electrically operated through control lines integral with the hoisting cable.
In the hole, the sonde itself is perturbed by not only seismic waves propagating through the formation to which it may be locked at some depth station, but also by random noise such as rattling of the supporting cable and tube waves that travel through the borehole fluid. The various spurious wave fields set up resonances in the sonde in accordance with its parameters of mass, length and moduli of elasticity. Such resonances, if transmitted to the associated seismic sensor, obscure or badly distort the desired seismic signals. Therefore, the sensor must be acoustically isolated from the sonde itself.
In U.S. Pat. No. 4,578,785, assigned to the assignee of this invention, the sensor is secured to one leaf of a spider. While the sonde is being lowered into the hole, the spider and the sensor are nested inside the bottom of the sonde. At a desired depth station the sonde is locked in place; the spider is ejected from the bottom open end of the sonde to expand and lodge itself against the borehole wall, completely isolated acoustically from the sonde. A slack umbilical line is provided to transmit seismic signals from the sensor to the sonde and thence to a processor at the surface via a wire line that is integrated with the hoisting cable secured to the sonde. The disadvantage of that device is the complexity of the ejection/recovery mechanism installed in the sonde plus the possibility that the spider might break loose from the sonde if a tight spot is encountered in the hole.
U.S. Pat. No. 4,874,060 discloses a sensor probe that is mounted in a recess in a sonde that is, as usual, secured to the end of hoisting cable. At a desired depth level, an anchoring arm locks the sonde against one sidewall of the borehole. The sensor is mounted on an hydraulic piston that pushes the sensor laterally out of the side of the sonde to contact the same sidewall of the hole against which the sonde is locked. The sensor module is not acoustically isolated from the parasitic vibrations that may be set up in the sonde. The hydraulic piston and circuitry is complicated. In the case of a triaxial system, three separate piston units are needed. By reason of its configuration, the moving parts of the assembly, as disclosed in the patent, are open to the drilling fluid in the well bore; mud and solid particles suspended therein become lodged behind the mechanism to jam it.
Another probe-type downhole sensor is taught by U.S. Pat. No. 4,811,814. Here, the sonde is locked against one sidewall of the well bore at a desired depth level. A sensor, spring mounted on a probe, is poked out laterally against the opposite side wall. Although the sensor is partially isolated from the sonde by springs, the probe extension mechanism (of undisclosed nature) is necessarily complicated and subject to jamming by particulate matter in the drilling fluid. Furthermore, if triaxial response is required, three separate mechanisms apparently are necessary according to the disclosure. Although the sensor is spring mounted and hence, is partially decoupled from the sonde, no damping is provided. Accordingly, harmonics and subharmonics of sonde resonances may corrupt the sensor signal.
A sales brochure promulgated by Schlumberger advertises a Combinable Seismic Imager.TM.. This brochure illustrates a sonde that includes a spring-mounted sensor probe that can be pressed against the sidewall of a borehole by an undisclosed mechanism. As with any probe-mounted device that must be pushed out of a recess in a sonde, the probe-extension mechanism is necessarily complicated and is subject to contamination by drill-fluid-borne debris.
It is an object of this invention to provide a downhole sonde-mounted seismic sensor that may be firmly pressed against the sidewall of a bore hole without use of complicated probe-extension mechanisms, that will have three degrees of acoustic and mechanical freedom with respect to the sonde, that will be acoustically decoupled from the sonde and properly damped, that will be protected from contamination by drill-fluid-borne debris and that will include triaxial capability within a single compact module.